Video teleconferencing arrangements are known which convert analog video signals into digital data to be carried on telecommunications lines. For video teleconferencing, the coder portion of a video CODEC (COder/DECoder) converts an analog input signal into output binary bits. At the receiving end, a CODEC converts the binary bits back to analog signals. Due to the high bandwidth requirements for acceptable video quality of the received video signals, a plurality of DS0 telecommunications channels are required for each video signal.
For video teleconferencing involving several locations, a video "bridge" is known. In this arrangement, a central unit (serving as the bridge) receives video signals from each of the locations, and transmits video signals to all of the locations. This imposes a relatively expensive hardware requirement for the bridge apparatus, since it must be capable of simultaneously handling many different signals, and must be capable of re-broadcasting a given signal to each of the plurality of locations. For example, in the prior art, bridge boxes are used for CODECs at a cost of about $80,000. Each prior art bridge box is a single central unit which sends out a plurality of signals. In the present invention, the bridge is distributed over all of the commercially available dialing inverse multiplexer units (the commercially available dialing inverse multiplexer unit corresponds to that shown and described in the above-identified co-pending application having U.S. Ser. No. 07/906,324), and is sold under the commercial name Fracdial.TM.) at their separate locations. The prior art bridges are limited by their processing speed or by the number of ports built in. This in turn limits the number of ports which can be handled. Thus, in the conventional bridging apparatus, the total number of locations which can be handled is fixed by the bridging hardware itself.
It is a problem in the art to provide an apparatus and method for use thereof, which is not limited by the capacity of a single unit, but which provides a distributed conference network capacity. It is a further problem in the art to provide a conference network enabling communication with a plurality of locations in a network loop, in which each location can selectively serve to originate and terminate the network loop.
Digital communications can be carried on commercially available T-1 communication lines. Such communication lines are described in tariff #270 filed by AT&T in 1982, which covers High Capacity Terrestrial Digital Service (HCTDS). According to this tariff, a T-1 communication line has a data transmission capability of 1.544 Mbps. A T-1 frame consists of 24 8-bit DS0 channels. The T-1 transmission rate utilizing DS-1 signalling can transmit 8000 frames per second, at 193 bits per frame, which yields a transmission rate of 1.544 Mbps. In a T-1 communication line link, DS-1 signalling is used. According to this type of signalling, the 24 channels, each of which comprise separate data streams, are transmitted as a single frame. Each channel contains 8 bits, for a total of 192 bits per frame. One additional bit is used in each frame for synchronization purposes, and accordingly a frame is actually composed of 193 bits. According to this standard, the rate of 8000 frames per second can be transmitted.
In many telephone systems, "robbed bit" signalling is used, which further reduces the usable capacity of each DS0 channel from 8 bits per frame to 7 bits per frame, reducing the capacity of a full frame from 192 bits to 168 bits, and thereby yielding a useful transmission rate of 1.344 Mbps. A DS0 can accommodate 64 kbps of bandwidth (8 bits.times.8000 frames/sec). However, when "robbed bit" signalling is used to indicate on-hook and off-hook states, only 56 kbps of bandwidth is guaranteed to be switched for any DS0.
For higher speed transfer than that available by a single DS0 channel, it is known to employ a plurality of DS0 channels which are located together physically along the same telecommunications route. Such high data rate communications are needed by, for example, video teleconferencing applications. For such data communications, however, it has been necessary to co-route all of the plurality of DS0 channels to guarantee simultaneous arrival without differences in propagation delay along diverse routes.
A typical interface provided for a T-1 multiplexer to an end user is the V.35 interface. For video teleconferencing, a video CODEC is used for converting an analog input signal into output binary bits. At the receiving end, another CODEC converts the binary bits back to analog signals.
In practice, in known devices for sending large amounts of data, a plurality of contiguous DS0 channels must be used. In conventional long distance networks, each telephone circuit carrying the video telecommunications signal would travel by a different path. For example, a telephone communication between New York and California might travel via Atlanta or Chicago, and a large number of other switching paths are also possible. This gives rise to a synchronization problem when using a plurality of telephone channels to transfer large bandwidth data on a plurality of lower bandwidth lines. That is, since the transmitted data may be transmitted via different paths, different transmission times are involved, making it difficult to reassemble in real-time the arriving data into the original large bandwidth signal. This causes substantial delays to arise in setting up video conference calls, due to the necessity of waiting until the requisite number of contiguous T-1 communication lines have been obtained by the telephone company, as explained further below.
In the prior art, for high speed data transmission requiring use of more than one DS0 channel, a plurality of channels must be obtained by the telephone company which occupy consecutive multiplex timeslots. This solution, which is both relatively difficult to implement by the telecommunications company and relatively expensive to purchase, is well-known. In this type of service, a user communicates by telephone with the telecommunications company in advance, to obtain the video conferencing telecommunications service. After a wait of at least several minutes, and occasionally of one-half hour, the requisite number of physically contiguous lines are made available by the telecommunications company for use. This solution is relatively inefficient for the telecommunications company since it is relatively difficult to free up a plurality of consecutive timeslots. It requires an extensive search by the telecommunications company to obtain the requisite number of lines and to keep them clear for a predetermined or unknown length of time. Accordingly, even for a video teleconference of relatively short duration, for example several minutes, a minimum fee for one-half hour of telecommunications company service is often required at present. Additionally, because these lines are dedicated and can only carry the transmissions of the users involved in the video teleconference, the line charges themselves are relatively high.
On present telecommunications lines, a telephone call originating at a first location may be routed by any one of a relatively large number of different telecommunications paths. Once a call has been routed to a receiving facility, the routing is not changed. This establishes fixed delays once the telecommunications path has been established. However, the specific path obtained is unpredictable, and the length of the communications delay is determined by the path length as well as by other factors such as whether a satellite link has been included in the path. This problem of sending large amounts of data over a plurality of non-contiguous telecommunications lines (as opposed to the prior art use of a plurality of channels which must be obtained in advance by the telephone company and which occupy consecutive multiplex timeslots), has only recently been addressed by the development of a dialing inverse multiplexer, as follows.
One commercial device usable for high bandwidth transmission over a plurality of telecommunications lines, which does not require that the telecommunications lines be contiguous, and which serves as a dialing inverse multiplexer, is known commercially by the name Fracdial.TM.. The Fracdial.TM. device is manufactured by Digital Access Corporation of Reston, Va. This commercial dialing inverse multiplexer operates on all digital circuit-switched networks. These networks include private networks as well as all local exchange and inter-exchange carrier networks, and is compatible with digital switches including digital PBX's. This dialing inverse multiplexer operates by dividing high bandwidth serial data streams into multiple circuits, dialing those circuits across digital, circuit-switched networks, and resynchronizing and reforming them into a serial data stream at the terminating end.
The aforementioned dialing inverse multiplexer is capable of providing a bandwidth over an arbitrarily large or small number of channels up to the limit of the number of channels which can be handled by the switched data service being used, simultaneously dialing up a large number of channels for data transmission, automatically conducting a standard error rate test on each of a plurality of channels, taking out of service any channels which fail the standard error rate test.
Further, it is also a problem in the art to provide a method of using an improved dialing inverse multiplexer, to form a network or loop using telecommunication lines among a plurality of different locations each having a like dialing inverse multiplexer device.
Moreover, it is also a problem in the art to provide a method of using an improved ,dialing inverse multiplexer, to form a network or loop among a plurality of different locations each having a like dialing inverse multiplexer device, the network or loop requiring a bandwidth greater than the capacity of a single channel, without requiring use of consecutive channels, physically contiguous telephone lines, or dedicated lines.
Also, it is a problem in the art to provide a method of using a plurality of dialing inverse multiplexers to form a network or loop among a plurality of different locations to provide a distributed conferencing network capacity for carrying data.